The invention relates to a radiation-curable liquid resin for secondary coating of lightwave guides. The invention further relates to secondary coatings prepared from the liquid resin, and lightwave guides having such secondary coatings.
Typically, lightwave guides have two coatings: a soft primary coating and a hard secondary coating. Currently, primary coatings are predominantly radiation-cured urethane acrylates. Radiation-cured epoxyacrylates or urethane acrylates are predominantly used as secondary coatings. (See, for example: "Polym. Mater. Sci. Eng."Vol. 55, 1986, pages 36 to 539). Resins used as coatings must be radiation-curable so as to permit rapid and cost-effective curing. Such resins must also be liquid so that it is possible to process them essentially solvent-free in order to be able to meet environmental protection requirements.
The epoxy and urethane acrylates currently used as secondary coatings have a sufficiently high glass transition range (T.sub.g .gtoreq.+60.degree. C). Moreover, they are compatible with primary coatings based on urethane acrylate. However, in the manufacture of the coating, it is also important that the corresponding liquid resins have high radiation sensitivity in order to permit high curing rates.
"Chemical Abstracts" Vol. 106 (1987), page 7, No. 161368y, for example, discloses a coating material for optical glass fibers. The coating material is based on polyoxyalkylene and can function for manufacturing secondary coatings. This coating material contains oligomers which are obtained through the conversion of hydroxy-terminated aliphatic linear compounds such as polypropylene glycol, with diisocyanates and tris-hydroxyalkylisocyanurate-di(meth)-acrylates. Such resins do have more than two radiation-curable groups per molecule so that they should have a high curing rate. However, coupling on radiation-curable groups via bisurethane structures (by using diisocyanates) results in resins having a high viscosity. It is, therefore, required to add large quantities of reactive thinners to the known resins in order to be able to apply them as coating materials. This extra step entails additional costs and is also accompanied by other disadvantages such as high vapor pressure of the reactive thinners.
Accordingly, it is an object of the invention to provide a radiation-curable liquid resin which is suitable for the manufacture of secondary coatings of lightwave guides which have a glass transition range.gtoreq.+60.degree. C.
It is further object of the invention to provide a liquid resin having a high radiation-sensitivity and high curing rate.
It is a further object of the invention to provide a radiation-curable liquid resin having a viscosity which, at most, requires the addition of relatively small quantities of thinning agents when applied as a secondary coating.
It is a further object of the invention to provide secondary coatings which are compatible with the primary coatings (particularly primary coatings base on urethane acrylates) of lightwave guides.